Modern wireless communication, radar and radio frequency identification (RFID) systems often operate under full duplex operation. A wireless transceiver comprises of a local transmitter and a local receiver. Full duplex operation occurs when a local transmitter is actively transmitting RF signals during the same time that a local receiver is detecting RF signals and/or backscatter from the surrounding environment. The local transmitter and local receiver are typically in close proximity to one another and are often placed within a common enclosure. It is also desired to operate the full duplex system using a monostatic configuration, namely a configuration that uses a single antenna common to both the local transmitter and local receiver. In a typical transceiver, the transmitted and received signals are typically routed to and routed from the single antenna using a duplexing filter, circulator or directional coupler.
It is known that operation of the local receiver during the time that the local transmitter is transmitting creates receiver problems as the transmitter energy leaks, couples and/or reflects into the receiver resulting in corruption, distortion, saturation and/or desensitization within the receiver. In some cases, a duplexing filter may be used to isolate the transmitted energy from the receiver if the transmitter and receiver are configured to operate at two different frequencies that allow the duplexing filter to provide the required isolation between the local transmitter and the local receiver. If the system is designed to operate with the local transmitter and receiver using the same RF carrier frequency or with different transmit and receive frequencies that are close in RF carrier frequency such that the duplexing filter cannot adequately provide the required isolation, then a portion of the local transmitter's transmission signal energy will enter the local receiver and reduce the local receiver's performance.
A basic RFID transceiver is a system designed for full duplex operation using the same RF carrier frequency. Referring to FIG. 1, a simplified block diagram of a RFID transceiver 1 has a transmitter output port 2 for transmitting RF energy, i.e., a transmit signal 11, to a RFID transponder or tag 106. The transmitted RF energy may or may not be modulated with data. The transceiver 1 also contains a receiver input port 5 for receiving signals from the tag 106.
A circulator 3 functions to route the transmit signal 11 to the antenna 4, route a received signal 12 from an antenna 4 to the receiver input port 5, and provide some level of isolation between the transmit channel of the transmitter output port 2 and the receive channel of the receiver input port 5. The transmitted signal 11 leaves the antenna 4, and is received by the RFID tag 106. The RFID tag 106 consists of an antenna 107 and electronics 108 which may or may not contain an internal power source.
If an internal power source is not used within the RFID tag 106, then an RF signal received by the RFID tag 106, i.e., the transmit signal 11, is rectified and used to power the tag electronics 108. RFID tags that operate in passive or semi-passive mode typically do not contain an independent RF signal source therefore communication between the RFID tag 106 and the transceiver 1 occurs when the RFID tag 106 changes its reflection properties or backscatter. In this operation, the transmitter needs to be active during all tag-to-transceiver communications. It is under this full duplex operation that the receiver is required to recover encoded data from the backscattered signal during the time that the transmitter is transmitting its RF carrier into the surrounding environment. The backscatter signal is received by the antenna 4 and routed to the receiver input port 5 through the circulator 3. This full duplex transceiver configuration can also be used in many radar applications such as ground penetrating radar where the transmitter and receiver are operating with the same RF carrier and the receiver is required to recover reflections from targets in the environment while the transmitter is actively transmitting energy.
In any wireless transceiver, it is important that the receiver not operate in an undesired condition that will create corruption, distortion, saturation and/or desensitization within the receiver from any signal or signals coming from within the transceiver or the surrounding environment. For example, if a receiver front-end is driven into saturation from a high level RF signal that leaked, coupled or reflected from the transmitter of the transceiver, the receiver performance could be significantly degraded. Alternately, if the receiver operates with a high level front-end, then the down-converted intermediate frequency (IF) portion of the receiver will need to properly handle the resulting high level down-converted signal otherwise the receiver performance could be degraded.
In the case of a direct conversion receiver, the received signal is directly down-converted to baseband. For this type of transceiver arrangement, any signal that leaked, coupled or reflected from the transmitter will create a large DC offset at the baseband that could saturate the baseband amplifier and/or analog-to-digital converter and degrade receiver performance.
In a traditional full duplex transceiver using a single antenna there are four predominate RF signal paths, two paths are desired, namely the uplink and downlink communication paths, and two other paths are undesired due to leakage and reflections within the transceiver. FIG. 1 shows an example of the four signal paths within a full duplex RFID transceiver system. The desired transmitter-to-tag signal, or signal path, 11 is the forward communication link between the transceiver 1 and the RFID tag 106. The desired tag-to-receiver signal, or signal path, 12 is the reverse communication link between the RFID tag 106 and the transceiver 1. In full duplex operation, the forward link and reverse link are operating simultaneously and data modulation may occur on one or both paths.
In any practical system, a portion of the transmission signal emitted by the transmitter never reaches the antenna 4 and enters the receiver input port 5 through the circulator 3 by a leakage path. This undesired leakage typically occurs due to practical limitations in design of the circulator 3. These limitations create a first undesired path 13 from the transmitter output port 2 to the receiver input port 5. Additionally, a portion of the transmission signal is reflected from the antenna 4 due to mismatch between a transmission line impedance and the antenna's input impedance and results in second undesired path, or reflected signal 14. This reflected signal 14 enters the receiver input port 5 through the circulator 3. It is known that these undesired signals 13 and 14 will create problems if the energy level is high enough to cause corruption, distortion, saturation and/or desensitization within the receiver.
As an example describing how a receiver can be driven into a non-linear state from undesired signal paths, assume that a RFID system operating in the 902 MHz to 928 MHz frequency range has a transmitter output power of +30 dBm (1 watt) applied to the antenna. Also assume that the receiver front-end of the RFID transceiver has a compression point of +0 dBm (1 milliwatt). In order to maintain linearity in the receiver, the leakage and reflected signals must be below the compression point of the receiver front-end. Circulator manufacturers typically specify the leakage path 13 to be around 23 dB for junction-type circulators and 13 dB for lumped-element type circulators. Antenna manufacturers typically specify the return loss in the range of 10 dB to 20 dB (2:1 to 1.2:1 VSWR). In this case, the circulator leakage 13 allows a signal level of +7 dBm (5 milliwatts) to enter the receiver front-end using the junction-type circulator. This signal level will severely drive the receiver front-end into compression thus greatly reducing receiver performance. A lumped element circulator would further compress the front-end with a leakage signal as high as +17 dBm (50 milliwatts). For the case of an antenna with a 20 dB return loss, the reflection 14 results in a signal level into the front-end of +10 dBm (10 milliwatts), which also compresses the receiver and greatly reduces receiver performance. An antenna with a return loss of 10 dB would further compress the receiver with a reflected signal level of +20 dBm.
In order to maintain linearity of the receiver front-end, the isolation of the circulator would need to be greater than 30 dB over the full operating bandwidth. This isolation level is very difficult to achieve in a low-cost circulator. In addition, the return loss of the antenna would need to be greater that 30 dB (1.06:1 VSWR) which is also difficult to achieve over the full operating system bandwidth.
There are several techniques to overcome receiver saturation due to circulator leakage and antenna reflection. One approach that has been implemented in RFID and Ground Penetrating Radar (GPR) systems uses two separate antennas, one for the transmit channel and one for the receive channel. In this configuration, the two antennas can be separated a large physical distance in order to improve the isolation between the transmitter and receiver. A two-antenna configuration is less desirable than a single antenna system due to the increased physical size and higher antenna cost. In addition, a two-antenna system may result in reduced performance in a multipath environment.
In many RFID systems, it is often desirable to use Circularly Polarized (CP) antenna(s) attached to the RFID transceiver. The CP antenna effectively transmits and receives energy in all polarizations. As RFID tags typically have linear polarization, using CP antennas at the RFID transceiver would allow the RFID tags to be positioned with any orientation within the environment. There are numerous designs that can be used in a CP antenna including a microstrip patch, cross-polarized dipoles and quadrifilar helix. Circular polarization can be created with asymmetries in the antenna geometry or using a dual-feed antenna where each feed port is driven with a signal of equal amplitude and 90 degrees phase difference (quadrature).
In a full duplex transceiver operating using a single antenna, the leakage through the circulator and reflection from the antenna represent a technical problem to the performance of the receiver. This problem is addressed by the present invention.